1. Field of the Invention
The present invention relates to a single-layered electrophotographic photoreceptor, and more particularly, to an electrophotographic photoreceptor having high sensitivity, a low residual potential and improved cycling stability.
2. Description of the Related Art
In general, an electrophotographic photoreceptor includes a photosensitive layer, including a charge generating material, charge transport material and a binder resin, formed on a conductive support. As photosensitive layers, function-separation type photoreceptors having a laminate structure in which a charge generating layer and a charge transport layer are laminated, have been widely used.
Photoreceptors have been viewed as having a drawback due to reacting readily with active substances (i.e., corona discharge induced products) such as ozone and nitrogen oxides. However, in recent years, since single layered photoreceptors that can be manufactured by a simplified process have been shown to be advantageous due to effective chargeability and due to the generation of only a small amount of ozone during the positive corona discharge that occurs during the charging process, such photoreceptors have attracted considerable attention, and extensive studies are underway.
Representative examples of conventional single layered electrophotographic photoreceptors include a photoreceptor comprising a PVK/TNF charge transport complex as disclosed in U.S. Pat. No. 3,484,237, a photoreceptor comprising photoconductive phthalocyanine dispersed in a resin as disclosed in U.S. Pat. No. 3,397,086, a photoreceptor comprising a thiapyrylium and polycarbonate aggregate and a charge transport material dispersed in a resin as disclosed in U.S. Pat. No. 3,615,414. However, the disclosed photoreceptors do not have sufficient electrostatic properties and are considerably limited in selection of materials. Also, since such materials are harmful, the materials are not employed any longer.
Currently, single layered photoreceptors having a charge generating material, a hole transport material and an electron transport material dispersed in a resin, as described in Japanese Patent Publication 54-1633, have become the subject of development. Since such photoreceptors are functionally separated by materials used, charge generation and charge transport, a wide variety of materials may be selected. Also, since the concentration of the charge generating material may be reduced, the functional, chemical durability of the photosensitive layer may be enhanced.
Single layered photoreceptors that have been proposed to date exhibit a large residual potential and poor cycling stability, which may be caused by several reasons. That is to say, in a single layered photoreceptor, since charges are generated at a charge generating material uniformly distributed in a photosensitive layer, hole/electron injection and transport must be highly efficiently performed. In particular, electrons having low mobility are liable to remain at a low electrical field where transport efficiency is low. In a single photoreceptor, light absorption, i.e., light excitation, is maximum at the surface of the photoreceptor and is exponentially reduced inside the photoreceptor. Even if a charge generating material present in a deep portion of the photosensitive layer is excited to generate ion pairs, electrons are recombined with holes having migrated from a portion closer to the surface to then be neutralized. Thus, it is presumed that electrons do not substantially contribute to charge generation. Rather, charge generation is mainly undertaken by an electron generating material present at a portion closest to the surface of the photoreceptor.
A residual potential is definitely proportional to a distance between a residual charge in the photosensitive layer and a surface charge. Thus, the shorter the distance is, the more advantageous the residual potential characteristic becomes. Since the distance depends upon an average distance between dispersing particles, a charge generating material having many primary particles, that is, a high concentration, and a small particle diameter is advantageous.
However, in a single photoreceptor, increasing the concentration of a charge generating material may give rise to deterioration in chargeability, resulting in an increase in dark decay. Thus, it is more effective to uniformly distribute a charge generating material having a particle size as small as possible.
Phthalocyanine compounds are widely used as charge generating materials. However, it is known that the phthalocyanine compound exhibits quite different electrophotographic properties depending on the kinds of central metals or crystal forms. In general, various crystal forms are known with respect to the phthalocyanine compounds, and representative examples thereof include an alpha (α) form obtained by an acid-pasting method in which strong aqueous sulfuric acid is added dropwise to cold water to be precipitated, and a beta (β) form obtained by crystallization in a soluble solvent such as α-chloronaphthalene. However, the crystal forms are termed just for convenience' sake in view of the preparation method, but are not commonly applied to various phthalocyanine compounds having different central metals.
A compound having tetravalent titanium forming a double bond with an oxygen atom at the center of a phthalocyanine ring is termed titanylphthalocyanine, from which two kinds of stable crystal forms are identified through crystal growth using sublimation. Atomic arrangement of each crystal form was determined by X-ray structural analysis by W. Hiller et al. in Zeit Fur Kristal., 159 p. 173 (1982).
Japanese Patent Publication No. 61-217050 discloses a single layered photoreceptor in which α-crystal form titanylphthalocyanine obtained by hydrolyzing dichlorotitanium phthalocyanine by aqueous ammonia is dispersed in a resin, the crystal form of the titanylphthalocyanine being the same crystal form as that obtained by the acid pasting treatment and termed an α-crystal form. According to the X-ray diffraction spectrum, the crystal form falls into Phase II, which was studied by W. Hiller et al.
Japanese Patent Publication Nos. 62-229253 and 63-116158 disclose amorphous titanylphthalocyanine obtained by the acid-pasting treatment and exhibiting no X-ray diffraction spectral peak, and an electrophotographic photoreceptor using the amorphous titanylphthalocyanine. The amorphous titanylphthalocyanine has a very small primary particle diameter.
However, when such amorphous titanylphthalocyanine is used for a photosensitive layer, a residual potential is reduced, but dark decay becomes severe, resulting in poor chargeability and cycling stability.